Natural gas wells and oil wells are often located in remote “off-grid” locations. Connecting these off-grid locations to normal electrical power distribution systems can be difficult and thus portable sources of power are often used, which may not be economical.
To ensure proper operation and prevent the formation of ice-like hydrates within the piping and valves connected to oil and gas wells (especially at pressure-drop locations such as the wellhead chokes), methanol is injected down-hole or upstream of the choke, free water is then removed (separated), and additional chemicals are injected by a pump or pumps. Other chemicals injected include corrosion inhibitors, scale inhibitors, paraffin inhibitors, biocides, emulsifiers, and others, as typically required in both natural gas and oil production. The use of chemical injection pumps in these remote locations is referred to as their “field use”, or “use in the field”.
When a well is brought online it immediately goes into what is called a “drawdown condition”, which is an elevated level of production, and is the maximum that these wells will produce at any given time. During this time there is a requirement for proportionally greater chemical injection. It is therefore a requirement of injection pumps to inject the necessary the chemicals in remote off grid locations while also having capacity to address drawdown conditions.
Historically, pneumatic injection pumps have been used for the injection of these chemicals, and most injection pumps found in the field are still of the pneumatic injection variety. Pneumatic pumps are typically driven by one of two methods. The first method utilizes a conditioned production gas, wherein the gas is brought to a quality that can be used to drive pumps and instrumentation within the production unit (also be referred to as a “skid”). The second drive method uses bottled propane as a clean source of pressurized gas to drive instrumentation and pneumatic chemical injection pumps. To address drawdown conditions, the current standard in the field is to use a high volume pneumatic pump typically driven by propane. This propane is brought to the well-head in a liquefied form and then vaporized when used to drive the pump. However, after this gas has been used to drive the equipment in the skid, the used gas cannot be recaptured without extraordinary effort and expense, it is therefore vented to the atmosphere. Data suggests that pneumatic chemical injection pumps may be responsible for 60-85% of gas vented from skids. In addition to this resulting in wasted gas, the vented gas is harmful to the environment with data suggesting up to 19 times the carbon footprint of CO2.
The above environmental concerns have led to the use of alternate power sources for the chemical injection pumps, notably solar power. Though solar powered pumps have higher initial cost for implementation, they can have favourable payback periods due to the elimination of wasteful gas. However, the reliability of solar powered systems can be poor in these remote well-site conditions, and the cost associated with malfunctioning pumps and associated downtime is high. For example, if the solar powered pumps malfunction, a high producing well may freeze due to a lack of methanol injection. Bringing a high producing well back online after such freezing is costly.
Currently two forms of rotational solar powered chemical injection pumps are found in field use: either a high-speed or a variable-speed solar powered injection pump.
High-speed pumps are the most commonly used solar pumps in the field primarily because of their low cost. These pumps operate at one continuous speed and have two states, a full-speed state and a stopped state, for example, using a 12-volt motor connected to a small offset cam drive with the motor mounted horizontally and a cam drive spinning vertically. The stroke of the pump delivers a few cubic millimeters per stroke, but the stroke rate is equal to the rotational speed of the motor, which can be as high as 1750 rpm. Because of this high speed a substantial amount of chemical can be injected prompting the need to turn the pump on and off continuously. However, cycling the electric motor from an off state to a full speed state in this way induces inrush electrical current.
In the case of electric motors with one speed, inrush can be 10 to 30 times the steady state running conditions. For solar powered pumps this is damaging to the life of the batteries used to drive the equipment. As the temperature in the field drops, the temperature of the batteries also drops and suppresses the chemical reaction required for the batteries to deliver their rated amp output. The effect of inrush on batteries in low temperature conditions results in a significant drop in deliverable amp hours, which represents a proportionate drop in system design autonomy. For example, automotive batteries, which are the most commonly used in the field, are routinely damaged due to inrush and a large number of them are sent to be recycled, resulting in high operational costs.
Using a variable speed solar powered injection pump addresses the inrush issue. For instance, using a 3-phase 24 VDC variable speed motor can eliminate the inrush. However, the only products currently available are expensive and limited in their capacity to drive multiple fluid ends. This in turn results in a smaller volume output from the pumps, limiting its effectiveness in the field especially during drawdown conditions.
Current designs are also found to be limited in their ability to drive multiple fluid ends. Current pump designs with more than two fluid ends are seen to suffer from a significant drop in deliverable pressure and/or volume when compared to a single fluid end. This in turn limits the amount of chemical that can be injected per cycle from currently available injection pumps.
There exists a need for a cost effective and reliable chemical injection pump capable of meeting drawdown conditions to serve as an alternative to the above stated examples currently in field use. There is a further need for an economical and reliable chemical injection pump with a reduced carbon footprint, and which can reduce or eliminate the gas being vented to the atmosphere.